Get exterior ring of geometries, and put it in geojson object - Postgis - SQL

Question

I have been searching for a few days for a SQL query that would allow me to create a GeoJSON that would give me the outline of polygons in my database.

That first query is working correctly, it permits to get 1 Geojson Object with all the multipolygons in only one cell.

 SELECT json_build_object(
    'type', 'FeatureCollection',
    'features', json_agg(
        json_build_object(
            'type', 'Feature',
            'geometry', ST_AsGeoJSON( 
                the_geom
            )::json,
            'properties', json_build_object(
                'name', name
            )
        )
    )
)
FROM layer

Now i'm trying to get the same GeoJson object but with Linestring instead of multipolygons. It seems that the function ST_ExteriorRing() doesn't work with the function json_agg() ? I don't know what I have to change to make it works ... Here is the last query i tried, which doesn't work :

SELECT json_build_object(
    'type', 'FeatureCollection',
    'features', json_agg(
        json_build_object(
            'type', 'Feature',
            'geometry', ST_AsGeoJSON(
                ST_ExteriorRing( -- get the exterior lines of multipolygons
                    ST_GeometryN(
                        the_geom,
                        generate_series(
                            1, 
                            ST_NumGeometries(the_geom)
                        )
                    )
                 )
            )::json,
            'properties', json_build_object(
                'name', name
            )
        )
    )
)
FROM layer

If i suppress the json_agg() function it works fine, but it this case i don't get a single cell in output. If i supress the ExteriorRing() function it works also fine, but I have MultiPolygons and not LineString like i would like ...

Any ideas how to solve that problem ?

Answer 1

I found it months later, so here is the answer :

SELECT json_build_object(
    'type', 'FeatureCollection',
    'features', json_agg(feature)
) as geojson
FROM (
    SELECT json_build_object(
        'type', 'Feature',
        'geometry', ST_AsGeoJSON(
              ST_ExteriorRing(
                   ST_GeometryN(
                         ST_Union(the_geom),1
                   )
              )
        )::json
    ) as feature    
    FROM layer
) t